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11.3 Dissolving Grade Pulp

Additional post cleaning by means of a centricleaner further reduced the

amount of resins, thus improving viscose filterability. The particle spectrum in

the viscose demonstrates both a clear reduction in the particle volume and a shift

of the average particle size to lower values (Fig. 11.10).

The results confirm that reinforced oxidation, for example by ozone, contributes

to an improved dispersibility of pulp resins, and this is an important prerequisite

for the effective separation of these impurities. The efficiency of resin removal

can be further enhanced by additional post-cleaning operations.

Although a certain amount of pulp resin may be beneficial for subsequent processing

steps, by improving accessibility to the cellulose substrate, practical experience

has taught that the best way to control extractives is to take measures to keep

them at a low level [32]. When the resin content falls below a certain level, the

homogeneity of subsequent reactions may be impaired because of a lowered surface

activity. In this case, the addition of small amounts of synthetic surface active

agents overcomes that deficiency.

Residual lignin, brightness

The residual lignin content in dissolving pulps is generally very low. The kappa

number, which specifies the amount of oxidizable (by KMnO4) structures containing

double bonds in the pulp, is typically between 0.2 and 0.5 units which translates

to a residual lignin content of about 0.05% [33]. The main reason for aiming

at a low kappa number is the high demand on optical properties. Residual lignin

structures strongly contribute to yellowing of the cellulosic products. The highest

demands on brightness and brightness stability are given for viscose, lyocell, and

acetate pulps. Similar brightness levels are required for dissolving pulps converted

to cellulose ethers for application in foodstuff and pharmaceuticals. Residual lignin

Is, however, not the only factor determining the optical properties of cellulosic

substances. Therefore, the relationship between pulp brightness and brightness

of the final product is also dependent upon the processing conditions, especially

In the case of alkaline derivatization procedures (e.G., viscose, ethers). In industrial

operations using constant conditions, pulp brightness is clearly reflected in

the brightness of the final product.

Brightness – and thus residual lignin – is not a concern for pulps used for technical-

grade cellulose ethers (major applications: textile, paper, drilling muds, ceramics,

etc.). Nevertheless, bleaching to brightness levels of about 70–75% ISO is

necessary to improve pulp reactivity and prevent precipitation of lignin compounds

in subsequent processing steps.

The residual lignin is not only a concern for optical properties, but also governs

the processability of dissolving pulps. It is reported that viscose filterability (determined

by the clogging constant) gradually deteriorates when increasing the residual

lignin content from about 0.17 to 0.36% [34].

Inorganic Compounds

The presence of certain inorganic compounds such as silicates, Ca salts, and catalytically

active transition metal ions (Fe, Mn, Co, etc.) clearly impairs the filterabil-

1033

11 Pulp Properties and Applications

ity and spinnability of a cellulose spinning dope (e.g., viscose or lyocell type of

fibers). Moreover, pulp contamination with inorganic compounds leads to a gradual

clogging of the spinnerets, and this alters the uniformity of the fiber titer [13].

In particular, the cations Ca2+ and Fe2+, as well as silicates, are considered to be

detrimental in this respect. Although Fe(II), and to a lesser extent Cu(II), promote

light-induced yellowing, both cations are involved in detrimental degradation reactions

in the presence of hydrogen peroxide bleaching (Fenton-type reaction).

Thus, all necessary measures must be undertaken (acid wash, chelation stage,

etc.) to remove catalytically active cations.

Surprisingly, most of the harmful ash components are not distributed homogeneously

in the pulp, but are present as particulates in certain cell fractions, particularly

in the parenchyma cells [35]. Therefore, the only promising way to reduce

the amount of harmful ash components is an efficient mechanical pulp treatment

which applies combined pressure screening of unbleached pulp and centrifugal

cleaning after bleaching. This treatment ensures the removal of extremely small

debris such as sand, bark specks, and shives. Again, the best way to control inorganic

compounds is to reduce them to the lowest level economically feasible.

11.3.2.2.2 Macromolecular Properties

Molar mass, molar mass distribution

Since celluloses from natural sources and after chemical treatment are always

polydisperse, the determination of the average molecular weight (e.g., by viscosimetry)

is insufficient to predict specific product properties. Additional information

is provided by the measuring molecular weight distribution (MWD) of dissolving

pulps.

Measurements of MWD reveal a multimodal distribution for pulps produced

according to acid sulfite cooking, while the PHK pulps show a rather uniform

MWD. Dissolving pulps, being representative of various applications including

viscose, acetate and high-viscosity ether, are compared in Fig. 11.11.

The numerical evaluation of the MWD, as well as additional pulp quality parameters,

are included in Tab. 11.7. As expected, the sulfite dissolving pulps (viscose,

high-viscosity ether) reveal a rather broad MWD, as indicated by the high PDI.

This is also reflected in the higher amount of short-chain molecules (DP <100),

lower values for the alkali resistances, and the large difference between R18 and

R10 when comparing at a similar viscosity level. In the case of a high-viscosity

ether pulp, the comparison is not valid because the entire molecular weight is

shifted to higher values (Tab. 11.7).

There are many reports which confirm that the chain-length distribution in the

dissolving pulp is a crucial property in the production of rayon fibers [37]. The

short-chain molecules represent the weakest part in the fiber: the shorter the molecules,

the lower will be the number of molecules linking the crystalline regions.

Avela et al. were able to show that all strength characteristics are significantly

reduced with an increase in the low molecular-weight fraction [38]. Treiber

1034

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